A forerunner of applicant's invention is Pritchard's “Solid State Emulation of Vacuum Tube Audio Power Amplifiers” (U.S. Pat. No. 5,636,284). Pritchard describes the audio power amplifier prior art in terms of a typical configuration consisting of two triodes which form a differential amplifier phase splitter which drive two pentodes connected through a transformer to a speaker and a circuit providing feedback to the triode inputs.
Prichard identifies six features of this tube circuit of importance in emulating its performance with solid-state devices. The first feature is a bias-shifting capability of the input circuit which contributes the swirl of timbre of a decaying musical note.
The second feature has to do with the grid conduction of the pentodes which are driven by the high-impedance triode drivers. The grid conductance and the high-impedance drivers limits the drive capability of the pentodes at zero grid voltage which is particularly important when considering the variation of load lines.
The third feature has to do with the screen grids modulating the pentode current and causing a drop in the screen voltages. The screen voltage drop lowers the transconductance or gain of the pentodes producing the compression associated with tube circuits.
The fourth feature has to do with the slope of tube plate current versus grid voltage always increasing which results in ever-increasing distortion without clipping resulting in low-level signals being essentially distortion free and higher-level signals being distorted. If one wishes to maintain the classic tone, this distortion must be preserved because it blends into the clipping distortion.
The fifth feature has to do with the cathode biasing in lower-power amplifiers which has an effect similar to the compressive action of the screen-voltage variation but at much shorter time constants.
The sixth feature has to do with the character of the plate curves as altered by the screen resistors. A large round knee produces a softer clip than a sharp knee.
The object of Pritchard's invention was a solid-state amplifier which had the same six features as tube-type audio amplifiers.
The Pritchard invention incorporates a variety of distortion-producing elements into and around linear systems to produce tube-like “effects”. The embodiment shown in Pritchard's FIG. 4 is a good example. A linear power amplifier 46 of any convenient class or topology is shown surrounded by the embellishments needed to produce the desired result. The problem with this approach is that the many non-linearities are a challenge to control when attempting to exploit them in mass-produced equipments.
An important need left unsatisfied by Pritchard's invention is a plug-in solid-state module for expendable vacuum tubes which would provide a means for extending the lifetimes of existing tube amplifiers. In order to keep using their existing tube amplifiers as conventional vacuum tube audio power pentodes become less available, audio amplifier users will be forced to either retire the equipment, or make the change to a satisfactory vacuum tube substitute module.
Pritchard's solution is an all-new, non-modular, all-transistorized audio amplifier which mimics the performance of a tube-type audio amplifier. Pritchard's approach cannot be used to create a modular stand-alone power pentode substitute, and will not preserve the existing gear for the end user. Pritchard's approach doesn't solve their problem of finding an individual or set of replacement power output tube(s) or substitute modules for existing audio amplifiers.
The primary object of the present invention is to satisfy the need for robust and reliable vacuum pentode substitutes by vacuum-tube amplifier designers and end users. This situation has developed due to the dwindling number of good-quality receiving-tube manufacturers across the globe.
A second object is to provide a pentode substitute which meets or exceeds the pertinent electrical specifications of the target audio output pentode.
These objects are achieved by addressing the following deficiencies in the vacuum pentode output device art:                The necessity for a heater and high-temperature electron emitter structure and the slow exhaustion over time of the emitting surface of the vacuum tube's cathode;        The instabilities of the input bias voltage and currents resulting from vacuum failure or contamination mechanisms that corrupt or otherwise disturb a pentode's electrical operation;        The limited maximum signal current flow capability of conventional vacuum tube audio pentodes leading to the limiting of peak signal events and noticeably reduced dynamic sonic expression for the end users;        The high plate-to-cathode impedance of power-output vacuum tubes which limits the ability to drive output transformers and the tube-type amplifier's dynamic range and bandwidth;        The impossibility of adjusting the transconductance of vacuum pentodes at time of manufacture once the tube element assemblies are installed in their glass housings thereby preventing dynamic current matching of pentode pairs or multiples of pairs;        The impossibility of adjusting the input bias voltage sensitivity of vacuum pentodes at time of manufacture once the tube element assemblies are installed in their glass housings thereby preventing fine static current matching of pairs or multiples of pairs of vacuum pentodes;        The inalterability of the warm-up interval of 11 seconds for most vacuum output pentodes;        The necessity for the host amplifier to supply filament current thereby resulting in hotter running amplifier power supplies, and higher radiated hum fields.        